The stimulation of various cells of the body has been used to produce a number of beneficial effects. One method of stimulation involves the use of electrodes to introduce an externally generated signal into cells. For example, in connection with electrode-based brain stimulation techniques, the distributed nature of neurons may be responsible for a given mental process. Also, different types of neurons reside close to one another such that only certain cells in a given region of the brain may be activated while performing a specific task. Not only do heterogeneous nerve tracts move in parallel through tight spatial confines, but the cell bodies themselves may exist in mixed, sparsely embedded configurations. This distributed manner of processing is an issue in attempts to understand canonical order within the Central Nervous System (CNS), and can make neuromodulation a difficult therapeutic endeavor. Due to this architecture of the brain, there are issues concerning use of electrode-based stimulation which is relatively indiscriminate with regards to the underlying physiology of the neurons that they stimulate. Instead, physical proximity of the electrode poles to the neuron is often the single largest determining factor as to which neurons will be stimulated.
Electrode placement and mechanical stability can also be an important influence on the effectiveness of electrode stimulation since location often dictates which neurons will be stimulated, and flawed location/stability can result in lead migration of the electrodes from the targeted area. Moreover, after a period of time within the body, electrode leads frequently become encapsulated with glial cells, raising the effective electrical resistance of the electrodes, and hence the electrical power delivery required to reach targeted cells. Compensatory increases in voltage, frequency or pulse width, however, may spread electrical current and result in increases in unintended stimulation of additional cells.
In connection with work by the named inventor(s) of this patent document, recently discovered techniques allow for stimulation of cells resulting in the rapid depolarization of cells (e.g., in the millisecond range). One method of stimulus uses photosensitive bio-molecular structures to stimulate target cells in response to light. For instance, light activated proteins can be used to control the flow of ions through cell membranes. Ion channels and ion pumps are cell-membrane proteins that control the transport of positively or negatively charged ions (e.g., sodium, potassium and chloride) across the cell membrane. Ion channels play an important part of various animal and human functions including signaling and metabolism. Using optically responsive ion channels or pumps to facilitate or inhibit the flow of positive or negative ions through cell membranes, the cell can be briefly depolarized, depolarized and maintained in that state, or hyperpolarized. Neurons are an example of a type of cell that uses the electrical currents created by depolarization to generate communication signals (i.e., nerve impulses). Other electrically excitable cells include skeletal muscle, cardiac muscle, and endocrine cells.
Various techniques can be used to control the depolarization of cells such as neurons. Neurons use rapid depolarization to transmit signals throughout the body and for various purposes, such as motor control (e.g., muscle contractions), sensory responses (e.g., touch, hearing, and other senses) and computational functions (e.g., brain functions). Thus, the control of the depolarization of cells can be beneficial for a number of different purposes, including (but not limited to) psychological therapy, muscle control and sensory functions. For further details on specific implementations of photosensitive bio-molecular structures and methods, reference can be made to “Millisecond-Timescale, Genetically Optical Control of Neural Activity”, Nature Neuroscience 8, 1263-1268 (2005). This reference discusses use of blue-light-activated ion channel channelrhodopsin-2 (ChR2) to cause calcium (Ca++)-mediated neural depolarization, and is fully incorporated herein by reference. Other applicable light-activated ion channels include halorhodopsin (NpHR), in which amber light affects chloride (Cl−) ion flow so as to hyperpolarize neuronal membrane, and make it resistant to firing.